13:45 〜 14:00
[SSS06-11] Effect of large-scale fractures on high Vp/Vs anomalies in the subducting oceanic crust
★Invited Papers
キーワード：地震波速度、クラック、フラクチャー
Anomalously high Vp/Vs ratios have been observed in the subducting oceanic crust at various subduction zones. Such anomalies are generally interpreted as high crack porosity, saturated by water with extremely high pore pressure required to maintain the cracks open under confining pressure. Conventional interpretations are typically based on laboratory data measured on centimeter-scale samples. However, a quantitative evaluation of the fluid behavior should take into account the effects of spatial heterogeneity of the cracks and the scale-dependence of effective elasticity, since the observed seismic velocities may reflect macroscopic structure.
In this study, we investigated the spatial distribution of both microcracks and relatively large-scale fractures, which are invisible in laboratory samples, in the oceanic crust. Systematic ultrasonic measurements and X-ray CT data analyses were conducted on ~400 m continuous core samples collected from the Oman Drilling Project Hole GT3A, which drilled the upper oceanic crustal section of the Oman ophiolite. The depth variations in continuous P-wave velocity data were used to infer the microcrack distribution, while the abundance of millimeter-wide vein minerals identified in the X-ray CT core images was used to evaluate the fracture distribution.
Theoretical models were applied to the obtained crack and fracture distributions, revealing that the extremely high Vp/Vs ratios of 2.35±0.1 observed in the subducting crust (Audet et al., 2009) cannot be explained by microcracks alone, and the contribution of millimeter-wide fractures is essential. As rock permeability can strongly depend on the scale of hydraulic radius, the relatively large-scale fractures may play a crucial role in episodic fluid migration within the subducting crust.
In this study, we investigated the spatial distribution of both microcracks and relatively large-scale fractures, which are invisible in laboratory samples, in the oceanic crust. Systematic ultrasonic measurements and X-ray CT data analyses were conducted on ~400 m continuous core samples collected from the Oman Drilling Project Hole GT3A, which drilled the upper oceanic crustal section of the Oman ophiolite. The depth variations in continuous P-wave velocity data were used to infer the microcrack distribution, while the abundance of millimeter-wide vein minerals identified in the X-ray CT core images was used to evaluate the fracture distribution.
Theoretical models were applied to the obtained crack and fracture distributions, revealing that the extremely high Vp/Vs ratios of 2.35±0.1 observed in the subducting crust (Audet et al., 2009) cannot be explained by microcracks alone, and the contribution of millimeter-wide fractures is essential. As rock permeability can strongly depend on the scale of hydraulic radius, the relatively large-scale fractures may play a crucial role in episodic fluid migration within the subducting crust.